Central venous cannulation, with or without a flow-directed pulmonary artery catheter, is commonly performed in patients undergoing cardiac surgery to measure central filling pressure and cardiac output, and to administer medications and fluids. The complications of central venous cannulation are numerous and include malposition, arterial puncture, pneumothorax, hemothorax, chylothorax, extravasation of infusate, thrombophlebitis, and infection. We describe a single-lumen catheter that was placed through the hemostatic valve of a 9F percutaneous introducer, which inadvertently entered the left internal mammary (internal thoracic) vein. The current case is unique in that it was diagnosed by visualization of the catheter during surgical dissection.